The process of optimisation of protection is intended for application to those situations that have been deemed to be justified in the first place. The principle, with restriction on the magnitude of individual dose or risk, is central to the system of protection and applies to all three exposure situations. It is defined by the ICRP as the source-related process to keep the likelihood of incurring exposures (where these are not certain to be received), the number of people exposed, and the magnitude of individual doses as low as reasonably achiev­able, taking economic and societal factors into account. This process of opti­misation over several decades has resulted in substantial reductions of occupational and public exposures, and is key to the entire approach currently advocated for radiological protection. Essentially, it is always aimed at achieving the best level of protection under the prevailing circumstances through an ongoing, iterative process that involves:

(i) the evaluation of the exposure situation, including any potential expo­sures (the framing of the process);

(ii) the selection of an appropriate value for the constraint or reference level;

(iii) the identification of the possible protection options;

(iv) the selection of the best option under the prevailing circumstances; and

(v) the implementation of the selected option.

4.1 Dose Limits

Dose limits only apply to planned exposure situations (but obviously not to medical exposures of patients). For occupational exposure in planned exposure situations, the limit is expressed as an effective dose of 20 mSv per year, averaged over defined five year periods (in other words, 100 mSv in five years), with the further provision that the effective dose should not exceed 50 mSv in any single year. For public exposure in planned exposure situations, the limit is expressed as an effective dose of 1 mSv in a year, but in special circumstances a higher value could be allowed in a single year, provided that the average, again over defined five year periods, does not exceed 1 mSv per year.

The limits on effective dose apply to the sum of doses due to both external exposures and to committed doses from internal exposures arising from the intake of radionuclides. Occupational intakes may be averaged over a period of five years to provide some flexibility. Similarly, the averaging of public intakes over a period of five years would be acceptable in those special circumstances where averaging of the dose to members of the public could be allowed.

Dose limits do not apply in emergency exposure situations where an informed, exposed individual is engaged in volunteered life-saving actions, or is attempting to prevent a catastrophic situation. For informed volunteers undertaking urgent rescue operations, the normal dose restriction may be relaxed. However, responders undertaking recovery and restoration operations in a later phase of emergency exposure situations should be considered as occupationally exposed workers, and thus protected according to normal occupational radiological protection standards, and their exposures should not exceed the occupational dose limits. (Female workers who are pregnant, or are nursing an infant, should not be employed as ‘‘first responders” undertaking life-saving or other urgent actions).

Notwithstanding the basic scientific method adopted by the ICRP, its approach to the selection of dose limits necessarily includes societal judgments applied to the many and varied attributes of ‘‘risk’’. Not only would these judgments probably be different from one operational context to another within any given society, but they are also likely to differ from one society to another. Providing general guidance is therefore not that easy, and the ICRP makes it clear that it is for this reason that its guidance is intended to be sufficiently flexible to allow for national or regional variations.